ABTesting: Remove deprecated APIs

[paulb777]

Please remove deprecations in public headers in Firebase 7.

~/gh4/firebase-ios-sdk/FirebaseABTesting/Sources/Public/FirebaseABTesting (master) $ git grep -B5 -i deprecat
FIRExperimentController.h-- (void)updateExperimentsWithServiceOrigin:(NSString *)origin
FIRExperimentController.h-                                    events:(FIRLifecycleEvents *)events
FIRExperimentController.h-                                    policy:(ABTExperimentPayloadExperimentOverflowPolicy)policy
FIRExperimentController.h-                             lastStartTime:(NSTimeInterval)lastStartTime
FIRExperimentController.h-                                  payloads:(NSArray<NSData *> *)payloads
FIRExperimentController.h:    DEPRECATED_MSG_ATTRIBUTE("Please use updateExperimentsWithServiceOrigin:events:policy:"

[paulb777]

Closing since #6602 merged